The present invention relates to an improved method for generating data for use in calculating a desorption isotherm and a pore size distribution characteristic of an adsorbent material such as activated carbon, alumina or silica granules or the like.
In the past, such data as is needed to derive a desorption isotherm or a pore size distribution for an adsorbent sample was obtained by loading the sample with the adsorbate gas in a static environment. Since adsorption of a gas on surface releases heat, an inconvenient delay was required before equilibrium conditions were achieved to enable measurements to be taken in such static systems.
Generally, in a system where a carrier gas continuously flows over the sample, the heat dissipation problem is circumvented but other difficulties were thought to preclude the useful application of such flow systems for obtaining data for use in the calculation of the isotherm or a pore size distribution of a sample. In particular, it was thought to be disadvantageous to measure the initial loading of the sample and poor precision was obtained as the loading on the sample decreased to zero, that is, where substantially all of the adsorbed gas is removed from the surface of the sample. Another expected difficulty with the flow method involved the removal of the final traces of the adsorbate since the partial pressure would be very low and the time interval very long, thus making precise measurements very difficult to obtain.
The method of the present invention avoids the foregoing difficulties by loading adsorbate on a sample, and then sweeping it out in a constant flow of a carrier gas while monitoring the effluent gas composition with a suitable detector such as a thermal conductivity detector or, more preferably, a gas density balance which performs a linear detection over a suitable range. The effluent composition and sample temperature are recorded as a function of time to zero loading, corresponding to complete removal of the adsorbate gas. For strongly held adsorbates, complete removal is attained by raising the temperature of the sample gradually to increase the rate of desorption. Also, the sample is intermittently agitated to keep the loading uniform.
The data accumulated by the calibrated temperature and composition detectors can be fed to a suitably programmed computer. Then a backward integration of the composition (taking into account the carrier gas flow rate) from zero to any desired loading is employed to obtain the adsorbate loading corresponding to each value of the isothermal partial pressure. However, where the sample is heated during desorption, the partial pressures are measured and corrected to the isothermal partial pressures by well known equations based on the Polanyi adsorption potential theory. The data obtained can be readily employed in standard and well known calculations to obtain the pore size distribution for the sample. Thus, a practically continuous desorption isotherm is calculated from the non-isothermal data and application of the Polanyi adsorption potential theory.
As will be apparent from the following description, the difficulty of desorbing and of measuring the low partial pressures of a strongly held adsorbate at low capacities is avoided and the time of performing the analysis to obtain the data is greatly shortened.